1. Field of Invention
This invention pertains to a means to lower emissions of carbon monoxide and nitrogen oxides in lean, pre-mixed gas turbine combustors. Specifically, this invention employs a catalyst deposited on the inner surfaces of the combustor in the region of combustion.
2. Brief Description of the Related Art
The lean, pre-mixed combustor, also known in the art as a dry low NOX combustor is a combustor in which fuel is premixed with air prior to combustion to form a largely homogeneous fuel lean admixture having an adiabatic flame temperature less than about 3100xc2x0 F. (1700xc2x0 C.). This differs from a diffusion flame combustor where the fuel is injected directly into the combustion zone and mixed with air during combustion. As a result, combustion is essentially at the stoichiometric fuel air ratio with combustion flame front temperatures as high as 4000xc2x0 F. (2200xc2x0 C.). Unlike diffusion flame combustors, lean, pre-mixed combustors avoid stoichiometric combustion and are able to inherently achieve lower NOX emission levels. In both approaches the combustion products are modified by dilution air to achieve the desired turbine inlet temperature, however lower amounts are required in the premixed system.
To achieve single digit NOX emission levels in a lean, pre-mixed combustor requires operating at a flame temperature of the fuel and air admixture no higher than approximately 2900xc2x0 F. (or about 1600xc2x0 C.). Unfortunately, as the flame temperature of a fuel and air admixture is decreased to approximately 2800xc2x0 F. (or little more than 1500xc2x0 C.), typically combustion becomes unstable and high carbon monoxide emissions are generated. Thus, legal compliance requirements placed on both NOX and carbon monoxide make the operating window for a lean, premixed combustor quite limited, even operating at rich enough conditions where NOX levels are as high as 15 or 20 ppmv.
Accordingly, various types of independently controlled pilots are employed in lean, pre-mixed combustors to extend the stable operating window below 2800xc2x0 F. (1540xc2x0 C.) to minimize NOX emissions. However, if the pilot is a flame some NOX is produced by it and often there is little or no corresponding improvement in overall carbon monoxide emissions. Thus, there is a very small operating window in which both NOX and carbon dioxide emissions meet environmental regulations.
The lean, pre-mixed combustor art is familiar with staging of combustion to achieve low emissions over a wide engine operating range where lower turbine inlet temperatures are required. Staging, however, has practical limits both in terms of its ultimate ability to reduce emissions as well as the level of complexity introduced into the design of the combustor system. Even with this complexity, most lean, premixed combustors cannot reliably achieve ever lower standards for carbon monoxide and NOX emissions, for example below 15 ppmv.
The art is also familiar with the use of catalysts to both improve combustion stability and reduce emissions in combustors. As demonstrated by U.S. Pat. No. 4,603,547, a catalyst was applied to the inner surface of a diffusion flame combustor for the purposes of flame stabilization. The patent teaches that in the event that the primary combustion zone is extinguished a re-ignition of the combustor can be achieved if the rich fuel-air mixture can contact a sufficiently hot catalytic surface. The catalytic surface must be non-continuous so that the flame created by the contact of the rich fuel and air mixture to it will leave the liner wall and ignite the bulk combustor flow. The discontinuity in the catalyst coating is identified in those regions where film cooling of the combustor would be non-existent, the surfaces prior to or directly over the film cooling air inlets.
As taught by U.S. Pat. No. 5,355,668, a catalyst applied to a diffusion flame combustor, such as that of U.S. Pat. No. 4,603,547 should also tend to reduce unburned hydrocarbons and carbon monoxide emissions. The invention, however, teaches that the combustor is completely film cooled, due to the temperature of combustion, and that the flame, or reactants, contact the catalytic surface.
The present invention allows achievement of both lower NOX emissions and lower carbon monoxide emissions in lean, pre-mixed combustors. These reductions are possible in a lean, premixed combustor both with and without open flame pilots. The invention also provides a means to operate at leaner conditions if carbon monoxide emissions are the limiting factor in the design, allowing lower firing temperatures and the associated incremental NOX reduction.
It has now been found that catalytic coatings applied to interior surfaces of lean, pre-mixed combustors can significantly reduce both carbon monoxide emissions, typically by more than fifty percent, and NOX emissions, typically by more than five percent at a given lean operating condition with an equivalence ratio less than 0.65. In addition, in a lean, pre-mixed combustor utilizing a pilot flame, the catalytic coating will allow the pilot flame fuel flow to be reduced thereby reducing pilot flame NOX generation for a given combustor carbon monoxide emissions level and exit temperature.
In the present invention a catalyst is deposited on the inner surfaces of the combustor with particular attention to the areas of highest interaction with combustion gases, not the flame or reactants. For catalyst effectiveness, it is important that the catalyst be located within the combustion zone on the combustor wall in areas that are not blanketed by film cooling air. As the combustion zone and film cooling within the combustion zone are altered due to different operational conditions, it may be necessary to coat the entire combustor to assure that the catalyst at any given time is in an effective area. Backside cooled liner walls are preferred since such systems do not flow significant cool air on the flame tube side of the wall where the catalyst is applied.
While a lean, pre-mixed combustor that does not utilize film cooling is ideal for this invention, a total elimination of film cooling is not required. It is critical that if film cooling is employed, that the operational non-film cooled area, that is the area of the combustor not film cooled at an operational condition where NOX or carbon monoxide reduction is desired, be at least about 10% of the total with 40% to greater than 70% preferred.
In addition, it is critical to this invention that catalyst cooling, generally accomplished by backside cooling of the combustor wall onto which the catalyst is applied, be engineered such that the catalyst is maintained at an effective operating temperature. This temperature is at a minimum the threshold light-off temperature for the particular catalyst interacting with the particular fuel. Typical precious metal catalysts have minimum operating temperatures of approximately 400xc2x0 C. Thus, with metal liners it is desirable to place the catalyst on a thermal barrier inner coating (TBC) which lines the inner surfaces of the flame tube or combustor liner. The catalyst can be applied directly to ceramic combustor liners if so equipped.